dprantl

Because I was looking around the brains the other day and I couldn't find any relay with LEDs on it (or any relays around the brains, period).

Dan
'91 928GT S/C

Alan

Yes it should - it should be a clear plastic encased double relay attached by a bracket to the ECU mounting frame

Alan

Jim bailey - 928 International

I have seen a solid version as well but yes 89 > should be fitted . It shuts down 2 cylinders on each bank which correspond to one coil , one distributor cap .

dprantl

Anyone got a pic? Maybe I'm just being blind...

Dan
'91 928GT S/C

Ed Scherer

Crappy old photo from years ago. '90 S4. Located as Alan already described.

Johan928

Dan,
Sometimes the relay is by-passed and removed when the car shuts down 4 of the cylinders and the technicians cannot find the problem to make the car run right.
My 91GT is without this relay as well. Removed at some point by a dealer during its life is my best guess.
John

docmirror

Actually, the shut down is the batch fire signal for the injector set. Nothing to do with the ignition.

Jim bailey - 928 International

The relay is called the " supervis.-ignition circuit " on PET but yes it shuts down 1/2 the fuel injection when it thinks 1/2 of the igniton has failed. I usually refer to it as the ignition monitoring relay.

dr bob

I saw it referenced in a wiring diagram as EZK Protection Relay also.


It's alsoways nice to work to known standards. Especially when there are plenty to pick and choose from.

TheoJ

ok, one more addition: it does not shut down the batch fire signal, but the 12v power supply to the injectors. The injectors are grouped in two sets and each set has a 12v power supply routed through the relay. The grouping is the same as the ignition group per coil. Like Jim said: it is thought of as a way to prevent overfuelling when one half of the ignition fails. The other side of the injectors is tied together and fed to the LH, who swiches the signal to ground for a brief moment, thus opening the injectors.

If you want to loose the ignition monitoring relay, you basically do what the 1987/1988 have: a fixed 12v link. Porsche at some point decided to have a requirement to detect a failure in one ignition half. You don't want to keep fuelling when there is no ignition, and fuel will exit at some point, arriving in the hot CAT. So safety was on their mind I suppose.

Here's a good writeup from our friend Rich Andrade
http://jenniskens.livedsl.nl/Technic...4/MyTip460.htm

Bill Ball

Also, to be clear, it doesn't monitor the igntion system at all. It has no idea if there is spark or not at any of the cylinders. It monitors output of a thermocouple positioned under exhaust outlets on each of the heads (cyl 4 and 6 [correction - make that 8]; the position changed in 91 to cyl 3 and 7). When the thermocouples produce a ouputs that differs by a set amount, the system trips and does what Theo describes. It PRESUMES there is an igntion problem producing cylinders fed one side of the igntion that are colder than the other side and shuts off the fuel supply to the colder side. Cars made w/o cats have a bypass relay installed that jumps 2,4,5.

jon928se

Corrected that for you.

Bill Ball

Thanks...

Shane E.

And to be even clearer it has nothing to do with the '86.5!!

Bill Ball

Yep, you're allowed to have cat fires! H'man had one in an 86 he bought when one of the coils shorted and he continued to drive on.

I have mixed feelings about this system. I disabled mine 2 or 3 years ago after it tripped 3 times for no apparent reason in a month. I decided these were false alarms because as soon as I stopped the car and restarted, there were no further incidents for days to weeks. When it tripped the first time, I was not immediately aware of it. You'd think you would know something serious was wrong when you suddenly lose half your power. But I drove a couple of miles uphill noting that the car was downshifting when it wouldn't have before, but not yet understanding what was going on. I had no carpet on the floor as I was coming back from an open road race, and I remove those for the race. Finally, I looked over and saw the bright red LED. I pulled over, killed the motor, restarted and drove for several weeks w/o further incident, as I mentioned.

So, the lesson is, it is possible not to realize right away when you lose 1/2 the cylinders. Under that scenario a cat fire is a real risk. After my first experience, I think I am sensitized enough that I would pick up the problem. Since the system only monitors 2 cylinders, neither it nor I would pick up a failure involving, say, bad wires to a few other cylinders, but I gather Porsche thinks that only a bank failure is significant enough.